1. Technical Field
This invention relates generally to controllers for cross-point switches and, in particular, to electronic controllers for high-speed switching.
2. Prior Art
Cross-point switches have found wide use in communications systems, particularly in the telephone industry. Shown in FIG. 1 is a simple one-sided cross-point switching matrix which switches four data lines 10, 12, 14 and 16. These data lines are connected through terminals to internal row lines 18, 20, 22 and 24 of the switch. In a semiconductor cross-point switching matrix, these internal lines are conducting paths. The switching matrix also has internal column lines 26 and 28 which cross the rows without touching or contacting them. However, at each intersection of a row and column is placed a bi-directional switch or cross-point, for instance cross-point 30 at the intersection of row 10 and column 26. Each cross-point serves to connect a row to a column. In the simplest case of a one-sided switch, the columns 26 and 28 are not connected outside the switch but serve only as internal conduction paths. All data enters and leaves the switch on the data lines 10-16. To accomplish this connection, cross-points need to be turned on. For example, if lines 10 and 14 are to be interconnected, then both cross-points 30 and 32 can be turned on, with column 26 serving as the internal connection. It can be readily seen however that the same interconnection can be made by turning on switches 34 and 36 which use column 28 as the internal connecting line. The cross-points are all controlled by a controller 38 which is connected to the switch by a control bus 40.
Many types of controllers have been described which may be used with the cross-point switching matrix of FIG. 1. With the advent of semiconductor cross-point switches and the popularity of electronic switching systems, many of the recent controller designs have been implemented in integrated circuits, or a programmable computer has functioned as the controller. Examples of computerized switch controllers are given by Anderson in U.S. Pat. No. 4,331,956, by Bulfer in U.S. Pat. No. 3,935,394, and by McEowen et al. in U.S. Pat. No. 4,196,316. However, almost all of these electronic switching networks have been designed for communication systems in which the switching can occur at a fairly leisurely pace because, once the interconnections have been established, the data lines remain connected to each other for substantial periods of time. However, in the case where the cross-point switch is to be used within a computer or other computerized system and the interconnections are generally short-lived, a controller which requires a long time to establish the interconnections will severely reduce the effective bandwidth of the cross-point switch. Alternately stated, from the time a data line interconnection is requested until it is accomplished, no data can flow through the cross-point switch, and, accordingly, the throughput of the associated computer is reduced. Although a computerized controller will operate relatively fast, it requires a number of machine cycles to effect the connection after the service request has been made and suffers a corresponding delay. This delay is particularly significant for a one-sided switch, such as that shown in FIG. 1, because of the multiplicity of possible connection paths, i.e., a column not presently in use must be chosen.